Sensors used to monitor exhaust of a modern internal combustion engine are subject to stringent customer requirements and severe operating conditions which drive sensor design and material considerations. Requirements include limitations on space volume, sensor operating temperatures ranging from −40 C to greater than 1000 C, operating vibrational inputs in the range of 0 to 2000 Hz and 29 g's of force.
A typical sensor includes a planar sensing element contained in a sensor body or shell. A wiring harness is attached to the sensor, for connecting a plurality of lead wires to the sensing element. A modern sensor, e.g. a sensor capable of monitoring exhaust gas constituents including NO and NO2, may require eight lead wires to the sensor element. These lead wires supply electrical energy for heating the sensor and gather signals from the sensing element, and typically interface with an electronic controller. Such signals from the sensing may include air/fuel ratio, and concentrations of various exhaust gases, including NOx.
Skilled practitioners are continually faced with the challenge of designing a connection scheme that connects the plurality of wiring harness lead wires to the sensing element, meets the stringent customer requirements for packaging size, and is robust to the severe operating conditions over the life of the engine. Another challenge in system design includes a need to prevent crossover ‘talking’ or interference between the various electrical signals, to ensure that signal integrity is maintained for the each of the outputs from the sensing element. There is a need to reduce errors during manufacturing and assembly of the connection scheme, and to make the various components robust to minor deviations in manufacturing tolerances.
One design constraint of importance is the diameter of the lead wires, compared to the sensing element. The sensing element size and design is driven by durability, signal integrity and response time, element warm-up characteristics, packaging constraints, and other concerns. A typical sensing element has a width of four to six millimeters, whereas typical lead wire is twenty-two (22) gage meaning a nominal diameter of 1.5 millimeters, based upon concerns for durability and strength. When twenty-two gage lead wire is used, the sum of the diameters of four wire leads (typical for a stoichiometric sensor) is six millimeters, which equals the width of the sensing element. Such density of lead wires introduces problems of mechanical and electrical interference between adjacent lead wires, and may interfere with the ability to attach wires to the sensing element. This has been addressed by placing contact pads on both sides of a planar sensing element to reduce lead wire density. However, a six millimeter wide sensing element having eight contact pads requiring attachment of eight wire leads ends up having the same problem of lead wire density.
Therefore what is needed is a scheme to connect a wiring harness to a sensing element, especially a planar sensing element with eight lead connections, for use in an engine exhaust environment that is robust to extreme temperature and vibration inputs over the life of the engine, meets stringent customer requirements, is robust to variations in the manufacturing and assembly processes, and maintains signal integrity.